The objective of this study is to assess tracking of bone mass during childhood and adolescence. Specifically, we will estimate the degree of tracking over 1 and 2 year intervals in girls between 3 and 17 years of age. Annual measurements bone mineral content and density will be measured by dual energy x-ray absorptiometry. A total of 300 girls will be followed for 3 years. To date, baseline measurements have been obtained on 229 girls. This study will lay the foundation for a longer- term longitudinal study of tracking in bone mass in children.